The invention preferably relates to a sheet of paper which includes at least one area having a uniform average opacity which is at least 40% lower than the opacity of the rest of the sheet.
In the present description, xe2x80x9cpaperxe2x80x9d indicates any sheet obtained in wet fashion with the aid of a suspension of natural cellulose fibers and/or synthetic fibers which can contain various fillers and various additives currently used in paper-making.
Papers which have been made transparent are already known in the prior art. These papers are rendered transparent by, for example, chemical compositions (see in particular the French patent No. 82 05124, filed Mar. 15, 1982 by Arjomari-Prioux). These papers are rendered transparent across their entire surface and are intended for applications in engineering drawing, plan reproduction, etc.
Chemical processes for rendering papers partially transparent, that is in determined places, are also known. These processes use grease-based or mineral oil or vegetable oil-based compositions which are imprinted. However, it is difficult to print on these transparent areas using the usual current printing techniques (offset, intaglio, heliography, etc.).
It is also known to produce thickness and density differences in the paper, particularly by means of the watermark technique used to impart security to papers for banknotes, identification papers or valuable official documents. These watermarks are obtained during the manufacture of the sheet of paper by means of cylinder molds which include sunken or raised imprints or with the aid of watermark rolls which include sunken or raised patterns associated with a flat table (Foudrinier machine). An image is then obtained which, when the sheet of paper is observed by transmitted light, appears either light, if the watermark roll includes a raised pattern, or dark, if the watermark roll includes a sunken pattern. The light areas are due to the fact that the sheet thickness and the fiber density are lower than the sheet thickness and the fiber density in the areas in which the watermark roll has not made an imprint. On the contrary, the darkened areas are due to the fact that the sheet thickness and the fiber density are greater.
Such watermarks include light parts which generally have surface areas of several mm2, and the variation in opacity between the light part of the watermark and the unwatermarked paper is slight. If it is desirable to print on the light areas of these watermarks, it is very difficult to coordinate the light areas with the printing, and even with very precise positioning of the watermarks, it is difficult to obtain good coordination.
European patent application No. 388 090, filed Mar. 9, 1990 by De la Rue Company, Plc., describes a security paper which includes areas of reduced opacity in predetermined, well-localized places, the areas making it possible to see by transmitted light, with the naked eye, imprints carried on the side opposite the side from which the paper is being looked at. The areas can be produced, for example, in the form of a watermark, using cylinder mold or watermark roll processes.
However, in this patent application, there is no description of how to obtain such areas from a manufacturing standpoint.
From the manufacturing standpoint, it is relatively easy to make paper which includes areas which are of reduced thickness and therefore light, obtained by means of the watermark technique, when the surface area of these areas is less than 0.4 cm2. On the other hand, if it is desirable to obtain areas whose surface area is greater than 0.4 cm2 by means of the watermark technique, one skilled in the art encounters technical problems.
The first problem is the following: In order to produce a watermark, one skilled in the art knows that it is possible to emboss the wire of a round cylinder with the aid of a punch. For example, if a circular light area is desired, the wire will be embossed with a circular punch. However, a light area with constant opacity is not obtained using such a process. In fact, it is noticeable that the fibers have a tendency to be deposited in greater quantity near the center of the embossed part than on the edges, and therefore the center of the watermark appears much less light than the edges. The same phenomenon occurs if a watermark roll which includes uneven raised portions is used.
A second problem which arises is that when areas of reduced thickness, which are therefore light, and which have a surface area of more than 0.4 cm2, are sought, no light area is obtained.
Various means for avoiding the falsification of checks are also known. Thus, it is possible to add chemical compounds which react with acids, bases, or pencil erasers into or onto the check paper. However, new anti-falsification means are continually sought.
The European patent application filed by the Applicant on Nov. 18, 1992, and published on Jun. 30, 1993 under the number EP-A-539 384, describes a process for manufacturing a sheet of paper which includes at least one area having reduced thickness relative to the thickness of the rest of the sheet, and having a surface area of at least 0.4 cm2, by disposing on an embossed or unembossed wire possibly disposed on a cylinder mold, an aqueous dispersion which contains at least cellulose fibers, by draining off the water in order to form the sheet by drying, with or without the presence of a watermark roll. Flexible pieces are used, which are associated with the watermark wire, the watermark roll or the round cylinder in such a way that the draining of the water during the formation of the sheet in the areas of the flexible pieces is reduced in relation to the draining of the water in the areas which do not include a flexible piece.
The sheet of paper obtained according to this process is such that the area having thickness is reduced to a thickness which is up to 40% lower than the thickness of the rest of the sheet. The sheet can also be such that this area has an average opacity which is up to 40% lower than the opacity of the rest of the sheet. Such a process makes it possible to obtain areas of reduced opacity. However, a reduction in the opacity of the areas is still being sought.
Moreover, the process requires a transformation of the paper-making machine, namely that the association of the flexible pieces with the watermark wire, the watermark roll, or the cylinder mold. These pieces must be fastened to the determined places and thus they require supplementary handling. Furthermore, the area has a thickness which is 30% lower than the thickness of the rest of the sheet.
Banknotes are also known which are constituted by sheets of transparent synthetic material on which a recto-verso impression is made, the impression being such that it leaves a completely transparent area. A hologram is transferred onto this transparent area. However, such plastic notes have several drawbacks. They are not very able to withstand handling and circulation since the impression can be fragile on the plastic. That is why paper notes are preferred, but at the same time why it is desired to produce the most transparent area possible. According to the process described in the document EP-A-549 384, the area obtained certainly has reduced thickness and opacity, but an area with even less opacity is sought.
Consequently, another object of the invention is to produce an area in the paper which is practically transparent or translucent, without piercing the paper in this area.
Another object is to produce an area in the paper which is practically transparent or translucent, this area being able to be printed on and/or being able to receive a hologram or any other distinctive mark or security mark.
This area must therefore be strong enough to support the transfer of a security mark.
Therefore an object of the invention is to furnish areas of reduced thickness relative to the rest of the sheet without having to use associated flexible pieces.
Another object of the invention is to furnish areas having a lower thickness reduced by much more than 40% relative to the rest of the sheet.
To this end, the invention relates to a sheet of paper which includes at least one area of reduced opacity, that is an area having a uniform average opacity at least less than about 40% relative to the opacity of the rest of the sheet, the area having a surface area of at least 0.4 cm2, characterized in that the area has reduced opacity due to the reduction of its thickness relative tot he thickness of the rest of the sheet and due to the fact that it has undergone a supplementary process to make it transparent.
The sheet according to the invention can include in a part of the area of reduced opacity, a hologram, an iridescent coating, an impression in standard inks or in inks which contain colored pigments, fluorescent or luminescent pigments, or photochromic, piezooptic or piezochromic compounds.
The sheet of paper can be a one-ply or monolayer sheet, that is a sheet which includes an area whose thickness is not nil, the residual thickness being made transparent chemically.
According to another embodiment, the sheet of paper can be a two-ply or double layer sheet, that is it is constituted by a first ply or layer which includes an area whose thickness is nil, and a second ply which is rendered partially or completely transparent. In the case in which the second ply is rendered completely transparent, this ply can be obtained according to the tracing paper technique, namely from a suspension of very refined cellulose fibers, in order to obtain the transparency. In the case in which the second ply is rendered partially transparent, it is made transparent chemically in an area which comes to correspond with the nil thickness area of the first ply.
The invention also relates to a process for manufacturing a sheet of paper which includes at least one region having reduced thickness relative to the thickness of the rest of the sheet, characterized in that:
a first layer of paper is formed on a wire of a first wet end of a paper-making machine,
a second layer of paper is formed on a wire of a second wet end of a paper-making machine,
one of the two layers of paper having at least some local regions of reduced thickness, so that the thickness of the two layers of paper in these regions is up to 80% of the thickness of the rest of the sheet,
the two layers are joined together and dried.
One or even two layers of paper can be produced without a problem, in such a way as to be very thin locally, at least one of the layers of paper in a region being locally reduced so that the thickness of the assembly of the two layers is reduced enough to obtain the proper transparency.
Even though the layers of paper are produced on separate paper wires, a sheet of paper is obtained which it is no longer possible to separate due to the rapid coupling of these layers and their being dried together. The sheet of paper is no different from a sheet of paper produced on a single wire.
The process is such that the thinner regions in the layer or layers of paper are produced by embossed regions on the mechanical wire, with the result that the deposit of fibers in these regions is reduced or completely prevented.
Preferably at least one of the layers of paper is produced by a cylinder mold machine.
Even more preferably, one of the two layers can include regions in which the thickness is nil.
The thinner regions of the paper webs are constituted in the form of a simple geometric motif, particularly in the form of flat regions which are approximately rectangular, round or elliptical, or in any other form.